Many of them are deliberately provocative to stimulate a response. Others are open-ended or neutral to allow whatever occurs in your mind to fit the topic. Use them for table topics at your Toastmaster's Club, with your public speaking class or by yourself.
Give yourself a strictly monitored preparation time of no more than one minute in which to note an opening, a couple of main points with examples and a conclusion.
Deliver your speech while timing and if possible, recording it. Aim for at least two minutes.
When you play it back, listen for fluency, structure and content. Each of those combine to create the overall impact of your speech.
If you're using these impromptu speaking topics with a friend or in a classroom setting, use the same method but omit the recording as you can give your feedback verbally.
Ideally once you or your class has become more confident, you'll lessen the preparation time.
The ultimate goal is to be given a speech topic and to begin speaking on the subject coherently and easily, almost immediately.
Literally, you get given the topic, the green light or timer is flicked on, you open your mouth and go!
If you've arrived at impromptu public speaking topicswithout having been to my impromptu speaking tips page, perhaps you'd like to go there to collect information on useful structural patterns and delivery tips before you begin. If you're new to this, they will help a great deal.
For further impromptu speaking practice ideas read my 'free word games' page. While the activities here are drama based you'll find them applicable and FUN! These are especially useful if you're a teacher looking for ideas to help you plan impromptu public speaking lessons for your class.